Transfer cases are often used in both full and part-time four-wheel drive vehicles to distribute driving power received through an input shaft from the vehicle transmission to a pair of output drive shafts. One of the drive shafts powers the vehicle front wheels and the other drive shaft powers the vehicle rear wheels. In vehicles permitting shifting between two wheel drive and four wheel drive modes, the input shaft of the transfer case provides continuous power to one of its output shafts and selectively provides drive power to the other output shaft by some type of disengageable or otherwise adjustable coupling, such as a viscous coupling, electro-magnetic clutch, or positionable spur gearing. Other drive modes are sometimes provided, including four-wheel drive high for higher four-wheel drive speeds, four-wheel drive low for lower driving speeds, neutral for disengaging the transmission from the front rear axles to allow towing, and locked four-wheel drive for controlling wheel slippage.
Additionally, other transfer case applications have evolved, such as on demand four-wheel drive, in which a transfer case, with its related parts that provide four-wheel drive, is installed in the vehicle, yet four-wheel drive mode is only engaged, by automatic means, when there is a loss of two-wheel drive traction. Full time or constant, four-wheel drive mode, sometimes referred to as “all-wheel drive” is also currently available in some automotive variants. In this mode, four-wheel drive is often not deselectable, and thus remains a fixed configuration.
In the transfer cases used for these vehicles, certain elements, or components, are required to transmit the driving force. More particularly, certain elements are required to selectively transmit the driving force during particular driving situations but not in others. One example of a device used to selectively transmit driving or rotational force, in a transfer case, is a one-way clutch. One-way clutches are known devices having inner and outer races with an engagement mechanism disposed therebetween. Generally speaking, the engagement mechanism is designed to lock the races together when the relative rotation of the races is in one particular rotational direction. When the races rotate in the opposite relative direction, the engagement mechanism is unlocked and the races have free rotation relative to each other. In application, when the races are fixed to concentric shafts, the one-way clutch will function to hold the shafts together when engaged, causing them to rotate in the same direction and thereby transferring motive force, or drive torque, from one shaft to the other. When the one-way clutch is disengaged, the shafts can then free-wheel relative to each other.
Specific applications govern how the one-way clutch engagement is designed. A one-way clutch may be designed to have one race as the driving member and one as the driven member, or the clutch may be designed to allow either shaft to act as the driving member during different operating modes. In this manner, the locking mechanism of the one-way clutch may be designed to engage in response to the rotation of only one of the races, or the clutch may be designed so as to engage if and/or as either race provides the proper relative rotation.
The one-way clutch is typically used in circumstances in which shaft to shaft, or shaft to race, rotational, torque-transferring engagements are desirable, but a “hard” connection such as a spline or keyed connection would not work. For example, during certain operating parameters, a four-wheel drive vehicle experiences driveline difficulties that arise from having the front and rear wheels cooperatively driven, which can be alleviated by the use of one-way clutch devices within the transfer case. When a four-wheel drive vehicle turns a tight corner with four wheels coupled together on a paved road, the vehicle may experience what is known as “tight corner braking effect”. This happens due to the inherent physical geometry that affects objects rotating at different radial distances from a center point. Two distinct effects generally occur with four-wheel drive vehicles. First, when any vehicle enters a curve the wheels on the outside of the curve must traverse a greater circumferential distance than the wheels inside of the curve due their greater radial distance from the center of the curve. The tighter the curve, the greater difference in the rate of rotational, angular speed between the inner wheels and the outer wheels. Therefore, in a curve the outside wheels must rotate faster than the inner wheels. This effect is exaggerated in a four-wheel drive vehicle but is generally countered by the differential assemblies of the vehicle installed at the front and rear axles. Secondly, since the front wheels are also leading the vehicle through the curve, they must rotate faster than the rear wheels. With a solid four-wheel drive engagement there is no device (such as a differential) to counter this action, and the slower moving rear-wheels act in an undesirable braking manner.
To resolve this problem, one-way clutches have been employed in the transfer case so as the vehicle begins turning a corner, the front wheels (connected to transfer case output shaft through a one-way clutch) are allowed to disengage and free-wheel faster than the rear-wheels. Specifically, the driven shaft of the one-way clutch (i.e. the output shaft to the four-wheel drive front wheels) begins turning faster than the input or driving shaft, and the locking mechanism of the one-way clutch disengages to allow free-wheeling of the output shaft relative to the input shaft. This momentarily takes the transfer case out of four-wheel drive mode, thus preventing the “tight corner braking effect”.
Another undesirable four-wheel drive driving effect occurs during engine braking in a manual transmission of a four-wheel drive vehicle when in four-wheel drive and coasting. The manual transmission maintains a physical connection to the vehicle engine, such that when the vehicle is allowed to coast, the engine places a decelerating or braking force on the transfer case, both input and output shafts, and ultimately on both front and rear wheels. The normal and undesirable parasitic affect of engine braking through the rear wheels of a manual transmission two-wheel drive vehicle has a negative impact on fuel consumption and efficiency, which is exacerbated in four-wheel drive vehicles by virtue of addition of the front wheels. Thus, when a one-way clutch is used in a drive line of the transfer case, the slowing of the input shaft through the engine braking effect allows the output shaft (connected to the front wheels) to disengage and freewheel, momentarily taking the transfer case out of four-wheel drive and preventing the engine braking effect from passing through the front wheels, reducing the negative impact on fuel efficiency.
Finally, in an on-demand application, a one-way clutch can be employed in the transfer case so that in a normal two-wheel drive mode, if one of the rear wheels should slip during vehicle acceleration, the rotating speed of the input shaft will increase, so that the one-way clutch engaging elements will bring the transfer case into four-wheel drive and the front wheels into a driven mode.
While proving to be of great value, as transfer case design technology utilizing one-way clutches has continued to evolve, one-way clutch designs have revealed certain limitations. Most importantly, while a one-way clutch could solve the above-motioned problems and disadvantages, the one-way clutch could only work in one direction, if used alone. In other words, the one-way rotational forward engagement between the input and output shafts in the transfer case could allow forward four-wheel drive movement, but not reverse four-wheel drive movement. To provide this functionality, additional mechanisms and devices are added to the transfer case to supplement the limited functionality of one-way clutches. However, this has added both weight and complexity penalties to transfer case designs.
Concurrent ongoing design goals of reducing the mechanical complexity and physical bulk of transfer cases while increasing their functionally has brought about the design of another torque transmitting device using the one-way clutch mechanism to allow engagement in a bi-rotational, or two-way, manner. This device is typically called a two-way clutch. A two-way clutch may solve all the above four-wheel drive difficulties while providing full forward and reverse functionality. It may allow the input shaft to be designed as a driving member for four-wheel drive modes, in both rotational directions while offering bi-directional free-wheel movement of the driven output shaft as needed when the input shaft is stationary or rotating slower than the output shaft.
Yet, even though conventional two-way clutch designs have been useful in solving these and other four-wheel drive issues, it has become apparent in applications that use a two-way clutch for a four-wheel drive engagement that certain deficiencies still exist. Specifically, there exists a physical angular distance from the engaged inner connection between the races of the two-way clutch for the first rotational direction to the engagement of the races in the reverse, or second direction. This angular distance also known as backlash, can cause mechanical problems as the two-way clutch is repeatedly called on to change its driving rotational direction over the service life of the transfer case. This is due to the mechanical load brought to bear in the switch from one rotational direct to the other. This rotational shift takes a form of a high-impact shock loading that is not only absorbed by the two-way clutch, but is also translated to the other components attached to a two-way clutch in the drive line, all to a repetitive detrimental effect. The shock loading is detrimental as it reduces component life and reliability, while adding unpleasant ride characteristics to the vehicle.
Some attempts have been made to reduce amount of backlash within two-way clutch assemblies. These attempts have generally required substantial, or radical, redesigns of transfer case structure. In the typical two-way clutch, the structurally inherent backlash can only be physically reduced to between about four and five degrees of rotation. Even this seemingly small amount of backlash causes the noted issues.
Therefore, there exists a need for improved clutch assemblies for use in transfer cases having reduced or minimal backlash, which can thereby reduce impact loading, extend clutch life, and improve riding characteristics of vehicles.